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British Medical Books Health Issues Computerized Tomography

Os Odontoideum


  • dens incompletely fused to body of C2
  • smaller than the normal dens but size may vary
  • it is located usually in position of the normal odontoid tip (orthotopic) or near the foramen magnum (dystopic)
  • often fixed to the anterior ring of atlas and the two move as a unit
  • instability common


  • rare - not known


  • unknown
    • failure of fusion of the base of odontoid
    • results from traumatic process

Associated syndromes

  • Morquio sy.
  • Multiple epiphyseal dysplasia


  • often asymptomatic & discovered incidentally
  • local neck symptoms (neck pain, torticollis, or headache)
  • transitory episodes of paresis following trauma
  • myelopathy (cord compression) or cervical & brain stem ischemia d/t vertebral artery compression (seizures, syncope, vertigo, visual disturbances)
  • sudden death


  • os odontoideum may be overlooked without tomograms
  • radiolucent oval or round ossicle with a smooth, dense border
  • moves with atlas in flexion & extension

Specific characteristics

  • gap between os & hypoplastic dens is wide & lies well above the level of superior articular facets of axis
  • not normal shape or size of dens - half size, rounded or oval with a smooth uniform cortex
  • if os is in area of foramen magnum - little dg. problem

Diff Dx

  • odontoid hypoplasia
  • non union


  • controversial

Indications for C1-C2 fusion

  • ADI > 10 mm or SAC < 13 mm
  • neurological involvement (even if transient)
  • progressive instability
  • isolated local symptoms (pain,torticollis) or brain stem symptoms (diplopia, nystagmus vertigo, dysarthria, dysphagia) are Not indications

C1-C2 fusion - Gallie type

  • followed by Minerva cast for 6-12/52